This invention relates generally to the pulverizing of particulate material. More particularly, it relates to an improved way of comminuting hard, pyrophoric and oxidizable rare earth metal alloys, such as samarium cobalt, iron, etc., which comminuted materials are particularly useful for producing uniform, high quality magnets.
Several known techniques exist for reducing the size of particulate. Often when using such reducing techniques, it is highly advantageous to control the size distribution of the comminuted material. This is particularly true when manufacturing rare earth magnets. In the manufacture of such magnets it is important to reduce the particles to approximately a crystallite having a single magnetic domain. This is due primarily to the fact that the presence of such single crystallites enhance significantly the magnets' properties. More particularly, a single domain particle is easier to magnetically align in a preferred manner than a particle containing several magnetic domains.
A conventional approach for reducing rare earth particulate used in making magnets is ball milling. Ball milling, however, suffers from several rather significant drawbacks. For instance, in the ball milling process, the steel balls erode and the powder has a tendency to gall (i.e., cold weld under high pressure and friction) to metal surfaces. Galling forms surface inhomogenities, which keep small powder particles from being crushed. Therefore, the powder quality is irregular. Ball milling also suffers from the fact that it is a batch process and, therefore, is not as desirable for production as a continuous method. To avoid overheating, galling, and oxidation, toluene is used. Consequently, the ball milled powder must be subsequently dried. The heating and transfer steps associated with the drying enhance oxygen pick-up. Furthermore, it is difficult to keep toluene free from oxygen and water. As a result, the powder oxidizes to a degree higher than that desired. Oxidation is detrimental to obtaining magnetic characteristics as high and uniform as possible. This is thought to be because the oxides of these rare earth powders have relatively poorer magnetic qualities than the non-oxide material. Moreover, for high quality magnets, the proportion of ingredients in such powders, as metallic samarium cobalt, should be uniform. Oxidation, however, reduces the desired proportion between, for example, the samarium and cobalt, thereby adversely affecting the resulting strength of the magnets. Aside from the above drawbacks, ball milling is costly and labor intensive.
Attrition milling is another known process for reducing the size of rare earth powders for use in making magnets. As with ball milling it is a so-called wet, batch operation. Accordingly, it suffers from some of the drawbacks mentioned above. Hammer milling is a known dry process, but is unsatisfactory for a number of important reasons. For instance, while there is a benefit in shattering the particles with a rotating hammer, there is a rather rapid wear of the hammer.
Jet or eddy milling is a continuous and dry method of comminuting the rare earth particulate. A jet stream carries particles for impact. However, such method is relatively slow and costly. The high purity gas used is expensive (e.g., 5-10 dollars per pound of particulate) or an expensive gas recirculating system is required. Also, in some mills comminution is obtained by particles impacting against each other which is an inefficient process. In others, the gas jet blows particles against an impact surface, where they shatter. Since smaller particles are accelerated by gas jets more than larger particles, the jet mills bias production of a rather broad particle size range towards smaller diameter while the impact speed of larger particles is smaller. Therefore, the comminution of larger particles is slow and inefficient.
From the foregoing it is apparent that the known processes for comminuting particulate, especially rare earth powders possess several distinct drawbacks.